Abstract:We demonstrate a new class of hollow-core Bragg fibers that are composed of concentric cylindrical silica rings separated by nanoscale support bridges. We theoretically predict and experimentally observe hollow-core confinement over an octave frequency range. The bandwidth of bandgap guiding in this new class of Bragg fibers exceeds that of other hollow-core fibers reported in the literature. With only three rings of silica cladding layers, these Bragg fibers achieve propagation loss of the order of 1 dB/m. References and Links1. P. Yeh, A. Yariv, and E. Marom, "Theory of Bragg fiber," J. Opt. Soc. Am. 68, 1196-1201 (1978). 2. J. C. Knight, J. Broeng, T. A. Birks, and P. St. J. Russell, "Photonic band gap guidance in optical fibers,"Science 282, 1476-1478 (1998). 3. R. F. Cregan et al., "Single-mode photonic band gap guidance of light in air," Science 285, 1537-1539 (1999). 4. J. C. Knight, "Photonic crystal fibres," Nature 424, 847-851 (2003). 5. Y. Fink et al., "Guiding optical light in air using an all-dielectric structure," J. Lightwave Technol. 17, 2039-2041 (1999). IntroductionThe concept of hollow-core Bragg fibers, in which the fiber cladding is composed of cylindrical dielectric layers with alternating refractive indices ( Fig. 1(a) -4]. In both cases, light is confined to a central hollow-core due to Bragg reflection from the periodic cladding structure. With little overlap between the propagating modes and the cladding materials, hollow core fibers are ideal for applications involving high optical powers [5,6]. Other potential advantages include significantly reduced nonlinearity and lower propagation loss [7,8]. In addition, hollow core fibers also provide an attractive paradigm to study novel nonlinear optical phenomena in gas phase materials [9,10]. One major challenge in fabricating Bragg fibers involves the identification of dielectric materials with not only a large index contrast, but also compatible rheological and thermal properties. In this paper, we report a new class of Bragg fibers that are composed of cylindrical dielectric layers in air, separated by nanoscale support bridges. Consequently, such Bragg fibers can be constructed from a single dielectric material such as a glass or a polymer. Furthermore, they have the unique property of supporting bandgap guiding over an octave frequency range, which is much larger than what has been achieved by any other hollow-core fiber [6,7,[11][12][13][14][15]. Ideally, our theoretical calculations indicate that such hollow-core air-silica Bragg fibers can achieve confinement loss of the order of 0.1 dB/km with only four silica cladding rings. This paper is organized as follows. In section 2, we theoretically analyze the properties of hollow-core air-silica Bragg fibers and demonstrate some of the unique features of this new class of Bragg fibers. In section 3, we discuss the fabrication and the experimental characterization of the hollow-core air-silica Bragg fibers in some details. The paper is summarized in section 4.

We investigate the connection between group velocity and rotation sensitivity in a number of resonant gyroscope designs. Two key comparisons are made. First, we compare two conventional sensors, namely a resonant fiber optic gyroscope (RFOG) and an interferometric fiber optic gyroscope (FOG). Second, we compare the RFOG to several recently proposed coupled-resonator optical waveguide (CROW) gyroscopes. We show that the relationship between loss and maximum rotation sensitivity is the same for both conventional and CROW gyroscopes. Thus, coupling multiple resonators together cannot enhance rotation sensitivity. While CROW gyroscopes offer the potential for large group indices, this increase of group index does not provide a corresponding increase in the maximum sensitivity to rotation. For a given footprint and a given total loss, the highest sensitivity is shown to be achieved either in a conventional RFOG utilizing a single resonator, or a conventional FOG.Unidirectional CROWs: Folded configuration, twisted configuration and planar configuration with alternating large and small rings.

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On the edge of the bandgap in a fiber Bragg grating (FBG) narrow peaks of high transmission exist at frequencies where light interferes constructively in the forward direction. In the vicinity of these transmission peaks, light reflects back and forth numerous times across the periodic structure and experiences a large group delay. Since the sensitivity of a phase sensor to most external perturbations is proportional to the reciprocal of group velocity, in these slow-light regions the sensitivity of an FBG is expected to be significantly enhanced over traditional FBG sensors operated around the Bragg wavelength. In this paper, we describe means of producing and operating FBGs that support structural slow light with a group index that can be in principle as high as several thousand. We present simulations elucidating how to select the FBG parameters, in particular index modulation, length, and apodization, to generate such low group velocities, and quantify the very large improvement in strain and temperature sensitivities resulting from these new slow-light configurations. As a proof of concept, we report an FBG with a group index of 127, or a group velocity of 2 360 km s. This is by far the lowest group velocity reported to date in an FBG. Used as a strain sensor, this slow-light FBG is shown to be able to detect a strain as small as 880 f Hz, the lowest value reported for a passive FBG sensor.